Frequency synthesizers have normally been fabricated to selectively combine a plurality of internally generated frequencies derived from a common source to produce an output signal which is adjustable over a given frequency range. Changing the synthesizer frequency involves switching from one set of input frequencies to another to obtain the desired output frequency. Such synthesizers have many varied applications. For example, they are used as variable frequency generators and as controlled local oscillators for receivers. They are also employed for programmable frequency applications. Frequency synthesizers may be used to provide coded frequency shifts in secure communication devices.
One type of frequency synthesizer is known as an indirect synthesizer and consists of a voltage controlled oscillator from which a signal sample is obtained and then divided down and compared with a fixed reference oscillator. The resulting difference signal is then applied to the control input of the voltage controlled oscillator resulting in a frequency change. Another divide and compare cycle is initiated with the difference signal again being applied to the voltage controlled oscillator control line. The frequency of the synthesizer is changed by changing the division ratio. This type of synthesizer utilizes time consuming counting techniques. For higher frequencies these counting techniques must be combined with frequency conversion techniques.
A second type of frequency synthesizer is known as a direct synthesizer and consists of a multitude of internally generated signals from a common high stability reference oscillator. The internally generated signals are selectively combined to produce the desired output signal frequency. The synthesizer frequency is varied by selecting different combinations of internally generated signals. This type of frequency synthesizer is complicated and has a restricted upper frequency limit.
In view of the above problems there exists a need for a frequency synthesizer that is relatively simple and is independent of external frequency references.